The optimization of the light outcoupling from an OLED is an important factor in the commercialization of OLEDs in many applications.
OLEDs have reached remarkable levels of power efficiencies over the years by lowering the operating voltage and increasing the quantum yield of internal light generation to almost 100% resulting in power efficiencies of more than 120 Lm/Watt for green light (Ultra High Efficiency Green Organic Light-Emitting Devices, Jpn. J. Appl. Phys. 46, p. L10-L12, (2007)) and recently 100 Lm/W for white light (Press Release of Jun. 17, 2008 by Universal Display Corp.).
The high efficiency is a major advantage not only for the use of OLEDs for displays, but also for their use as a light source for signage and general lighting purposes. Nevertheless, the efficiency could be much higher if all the light which is generated within the OLED device were actually being coupled out of the device. Currently, this efficiency is only about 20-25%, due to the planar waveguiding properties of the OLED device.
Efforts have been made to enhance this light out-coupling by putting optical stacks and/or scattering layers on top of the device. However, these methods do not necessarily result in optimized light output.
OLEDs could play an important role as one of the illumination sources for the future because of their high efficiency and their unique ability to form a uniform diffuse emitting surface. Efficiencies of more than 100 Lm/Watt for white light are sought for. Surface areas for these new light sources might be between about 10-100 cm2 to several square meters.
The challenge in creating an OLED emitting white light is not only to create white light efficiently, meaning that the source should contain enough of the red, green, and blue component of the visible spectrum to be perceived as white, but also to create a good ‘color rendering,’ meaning that all the wavelengths of the visible spectrum should be well represented in the optical output. Ideally, the wavelength distribution would mimic that of black body radiation.
Unlike displays where a sharp emission in the red, green, and blue area of the spectrum is an advantage, the challenge is here is to create a broad emission spectrum covering the whole visible range.
Another desirable feature is that the color point of the light source should not change too much by looking at it at a sharp angle; it should not appear red or blue.
A multilayer mirror formed by a quarter-wave stack only enhances one narrow band of wavelengths of light emitted from the OLED. The structure in a quarter wave stack is wavelength dependent, which means that the use of the same structure for red, green, and blue (RGB) applications can be problematic. Also, the number of layers needed for an effective quarter wave (QW) stack can be quite high (e.g., 10 layer pairs or more), which makes it impractical for many commercial applications. Furthermore, the OLED emission is generally broad. In a well designed QW stack, it is possible that a portion of the light will not be transmitted. Another disadvantage of the structure is that the intensity of the light transmitted by a QW stack is strongly directional, and in general the stack is designed with maximum intensity for forward transmission. This feature is generally unfavorable for display applications because it limits the viewing angle and is unacceptable for general applications in solid state lighting (SSL). The implementation of effective QW stacks requires strict control of thickness uniformity (Δ t<5 nm). This is generally achieved with a thin layer (t<20 nm) deposited at relatively low speed. It would become impractical for thick layers (100 nm<t<1000 nm), as for example, the layers required to form an efficient environmental barrier.
Optical cavities (single and multiple) have been suggested to enhance the light output of OLED devices. Similarly to QW stacks, optical cavities are wavelength specific. Therefore, a specific design and its accurate implementation are necessary for each wavelength. The emission from a single optical cavity is very angular dependent. Double or triple cavities must be implemented to make the emission independent from the observation point. The implementation is therefore practically difficult and complex and may require a different sequence of layers as well as different thicknesses of the layers in parts of the device.
Because QW stacks and optical cavities are wavelength specific, they cannot be used with white light sources.
Another problem with using OLEDs in white light sources for general lighting, signage and backlight applications, and in flat panel displays and other information display formats is that they are limited by the poor environmental stability of the devices. G. Gustafson, Y. Cao, G. M. Treacy, F. Klavetter, N. Colaneri, and A. J. Heeger, Nature, Vol. 35, 11 Jun. 1992, pages 477-479. Humidity and oxygen significantly reduce the useful life of most OLEDs. As a result, these devices are typically fabricated on glass substrates with glass covers laminated on top of the OLED with the edges sealed to exclude water and oxygen from the active layers.
Thus, there remains a need for a method of improving the light outcoupling of encapsulated white OLEDs compared to the bare white OLEDs, while protecting the OLED from environmental contaminants.